When Byzantine Emperor appealed to him, Pope Urban V sent out a call to the crowned heads of Europe. A Crusade against Turks was assembled with King Asmodeus of Savoy as leader who came with a fleet and an army. When routed by the Turks, the Balkan powers joined together and under the command of the King of Serbia attacked Turkish positions. A battle was fought in 1371, in which the Serbs and their allies were defeated and Murad annexed Macedonia. Murad now sent raiding parties even to Albania and Greece and became so powerful in the Balkans that John Palaeologus, the Byzantine Emperor at last became a vassal of the Turkish Sultan.

The Bulgarian Czar, Shishan II, even though he had allied himself with Murad by giving his sister in marriage, was forced by the remaining Balkan states, the Serbians and Bosnians, to make one more effort to stop the advance of the Turks. He raised a large army and the Christians allies won a battle at Vedin but were made to retreat soon afterwards. Now came another alliance and all the Balkan states under the command of King Lazar of Serbia made a combined attack. A great battle was fought at Kossovo on 20 June, 1389. The Turks broke the ranks of allies and inflicted a crushing defeat on them. Prince Bayazid succeeded Murad and following up the victory of Kossovo, he compelled the King of the Serbs to ask for peace. He entered Wallachia, now Rumania, and made its Prince to pay him tribute. The whole of Bulgaria now came within the growing Turkish Empire.